What is a Phototransistor?

A phototransistor is a bipolar junction transistor (BJT) that combines the concepts of both photodiodes and transistors into a single device. A photodiode is a semiconductor device that converts the incident light into an electric current known as photocurrent while a transistor is a device that regulates & amplifies the voltage or the current flow through it.

The collector-to-emitter current flow in a normal transistor depends on the base current present in it. For a phototransistor, the photocurrent generated in the base region due to the incident light acts as the base current and regulates the collector to emitter current. This device is ideal for photodetection, electronic switching, and current amplification applications.

Like normal transistors, a phototransistor has three regions or layers and two junctions. These are the emitter, base, & collector regions and the junctions are along the emitter-base & base-collector interfaces. These junctions are known as depletion regions or space charge layers (SCL) and they contain an electric field as electrons and holes are separated in these regions.

The emitter and collector regions have their respective terminals and are connected to the bias voltage. As the photocurrent acts as the base current the phototransistor does not require a base terminal but has a larger area for the base region to collect incident photons.

The photon incident on the phototransistor is absorbed in the base-collector junction to generate an electron-hole pair. The electric field present here will cause both the electron and hole to drift away in opposite directions; the electron moves towards the collector terminal while the hole moves towards the base region; thus producing the base current which causes the electrons in the emitter region to be injected into the base region. This results in the formation of an amplified collector-emitter current depending on the current gain of the device.

Normally, transistors are enclosed in metallic or plastic packages as they are light-sensitive. However, a phototransistor is designed to operate under light exposure and hence uses transparent packaging. It has a larger area for the base region & collector region and their junction is optically accessible for the incident light. Also, this device is designed to have high light sensitivity.

The symbol used for a phototransistor in the npn configuration is shown in the above figure.

Phototransistor as a current amplifier

Various circuit configurations can be implemented using a phototransistor. Among them, common emitter and common collector configurations can act as amplifier circuits 

A phototransistor in the common emitter configuration has a load resistor connecting the supply voltage to the collector pin. The output for this circuit is taken from the collector. For the common collector configuration, a load resistor is used to ground the emitter region. The output for this circuit is obtained at the emitter pin.

In the common emitter configuration, when the incident light is absent, the photocurrent or the base current is effectively zero and hence the phototransistor will be in the ‘off’ state. So, the applied bias current will be available as output. As the incident light increases, the photocurrent generated will increase; this in turn will be amplified and hence the current flowing through the phototransistor from the collector region to the emitter region will increase. As a result, the collector output will reduce.

In the common collector configuration, when the incident light is absent, the photocurrent or the base current is effectively zero and hence the phototransistor will be in the ‘off’ state. So, the output at the emitter load will be effectively zero amps. As the incident light increases, the photocurrent generated will increase; this in turn will be amplified and hence the current flowing through the phototransistor from the collector region to the emitter region will increase. As a result, the emitter output will increase.

A phototransistor can be used at low light levels as a current amplifier. It can amplify the current flow based on the photocurrent generated from the light incident on the base-collector junction. A suitable collector load is generally used to make the amplification linearly proportional.

Phototransistor as a switch

When there is little or no incident light, effectively no current will flow in the transistor, and this state can be defined as an "off" state. As the incident light on the phototransistor increases, the current flow through it also increases. Eventually, the phototransistor reaches a saturated state where the amount of current flow cannot be further increased. This state can be defined as the “on” state. So, by varying the incident light intensity, a phototransistor can operate as an electronic switch. 

Phototransistors are also used in optoisolators, photo-interrupters, proximity detectors, counting systems, encoder sensors, printers, optical control remotes, light detectors, level indication & relay systems, and alarm systems.